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Date:   Mon, 29 Mar 2021 16:27:06 +0200
From:   Oleg Nesterov <oleg@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Marco Elver <elver@google.com>, alexander.shishkin@linux.intel.com,
        acme@kernel.org, mingo@redhat.com, jolsa@redhat.com,
        mark.rutland@arm.com, namhyung@kernel.org, tglx@linutronix.de,
        glider@google.com, viro@zeniv.linux.org.uk, arnd@arndb.de,
        christian@brauner.io, dvyukov@google.com, jannh@google.com,
        axboe@kernel.dk, mascasa@google.com, pcc@google.com,
        irogers@google.com, kasan-dev@googlegroups.com,
        linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        linux-kselftest@vger.kernel.org, Jiri Olsa <jolsa@kernel.org>
Subject: Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events
Message-ID: <20210329142705.GA24849@redhat.com>
References: <20210324112503.623833-1-elver@google.com>
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On 03/29, Peter Zijlstra wrote:
>
> On Thu, Mar 25, 2021 at 09:14:39AM +0100, Marco Elver wrote:
> > @@ -6395,6 +6395,13 @@ static void perf_sigtrap(struct perf_event *event)
> >  {
> >  	struct kernel_siginfo info;
> >
> > +	/*
> > +	 * This irq_work can race with an exiting task; bail out if sighand has
> > +	 * already been released in release_task().
> > +	 */
> > +	if (!current->sighand)
> > +		return;

This is racy. If "current" has already passed exit_notify(), current->parent
can do release_task() and destroy current->sighand right after the check.

> Urgh.. I'm not entirely sure that check is correct, but I always forget
> the rules with signal. It could be we ought to be testing PF_EXISTING
> instead.

Agreed, PF_EXISTING check makes more sense in any case, the exiting task
can't receive the signal anyway.

Oleg.